Tin-Centered Radicals

In an esr investigation of Sj 2 reaction of various radicals on stan-nacycloalkanes (144) Davies assumed that a competitive process was the 

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The first tin-centered radical has been isolated, though it is not strictly organometallic in the C-Sn bonded sense. It is prepared by the reaction shown in Equation (173), in which the SnCl2 acts both as a substrate for forming the stannyl anion and as a one-electron oxidizing agent. 

The propagation steps for this reaction are outlined in Scheme 4.15. The tin-centered radical abstracts X from R-X, generating a carbon-centered radical, R , which subsequently abstracts hydrogen from the tin hydride, yielding the organic product, and completing the chain by regenerating the tin radical. 

This evident similarity of proton polarizations observed for 9 and 10 shows that, in the case of 9, XH CIDNP effects could only be employed to trace the fate of COCHMe2 and CHMe2 radicals and the pathways of formation of the reaction products that do not contain organotin function (Table 3). To facilitate analysis of the structure and multiplicity of the initial radical pair and the reaction pathways of tin-centered radicals, it is much more convenient to employ 13C and 119Sn CIDNP techniques. 

Photolysis of Sn[CH(SiMe3)2]2 yields the long-lived trivalent tin-centered radical shown in Eq. (43) (82, 83). 

If one of the reactions in a radical chain sequence is too slow to compete effectively with radical-radical reactions, the chain will collapse. Slow reactions of simple silanes such as Et3SiH with alkyl radicals precludes their use in the tin hydride method. Although quite reactive with alkyl radicals, thiols and selenols fail in the tin hydride method because the thiyl and selenyl radicals do not react rapidly with organic halide precursors. Nonetheless, it is possible to use thiols and selenols in tin hydride sequences when a Group 14 hydride is used as a sacrificial reducing agent. The thiyl or selenyl radical reacts with the silane or stannane rapidly, and the silicon- or tin-centered radical thus formed reacts rapidly with the organic halide [8], In practice, benzeneselenol in catalytic amounts has been used in radical clock studies where BusSnH served as the sacrificial reductant [9]. 

The availability of radical clocks that are a-substituted carbon-centered radicals or heteroatom-centered radicals is limited, however. Several experimental difficulties have limited progress in measurements of absolute rate constants for these types of radicals. One problem is the lack of precision for low-temperature ESR studies, and another has been a limited number of reactions available for production of radicals in LFP studies. A third fundamental problem affects the types of LFP studies described above for Bu3SnH specifically, the UV absorbance of the tin-centered radical is weak, and its signal can be obscured by absorbances of other species. 

A radical initiator, such as azobisisobutyronitrile (AIBN, 7) decomposes homolytically under reaction conditions to generate 8 which abstracts a hydrogen from tributyltin hydride (9) creating a tin-centered radical 10. The tin radical attacks at the sulphur atom of the thiocarbonyl derivative 2, generating intermediate 11 which collapses leaving tributyltin xanthate 12 (which can further decompose to 0=C=S and 13) and alkyl radical 14. Intermediate 14 can abstract another atom of hydrogen from tributyltin hydride to generate the desired alkane product 3, and replenish the pool of tin radical 10. 

The resulting tin-centered radicals can react with a variety of organohaUdes (or related compounds) to efficiently form carbon-centered radicals this type of reaction has played a central role in the development of synthetic and mechanistic organic radical chemistry. > Halogen-atom abstraction from alkyl or aryl halides allows the controlled formation of a variety of carbon-centered radicals, which can undergo a number of intra- or intermolecular carbon-carbon bond forming reactions. This includes cascade (or... 

It should be noted that the majority of radical reactions involving tin-centered radicals involve the use of tin hydrides, especially "tributyltin hydride, Bu,SnH. This reagent can be used to reduce a variety of organohahdes via radical intermediates, and the reactions can be initiated by photol) is of peroxides or azo compounds (chiefly azobisisobutyronitrile, AIBN). ... 

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